A marine propulsion system may have a propeller with variable pitch, meaning that the angle at which the blades of the rotating propeller meet the water can be changed. For reliable control there is needed a pitch angle indicator that provides a feedback signal indicative of the actual, measured pitch angle. The indication provided by the pitch angle indicator is typically transmitted in electric form for remote reading, but marine regulations may require a directly readable, mechanically produced visual indication.
Mechanically conveying a pitch angle indication signal from the propeller of a steerable thruster to inside the ship's hull is somewhat more complicated than in the case of direct shaft drive, because the propeller is on a horizontal shaft in the lower end of an azimuthally rotatable pod, i.e. a mechanical entity that turns about a vertical axis.
FIG. 1 is a simplified schematic illustration of a prior art solution for providing pitch angle feedback. A propeller 101 is installed on a horizontal propeller shaft 102. A vertical drive shaft 103 turns the propeller shaft 102 through a pair of meshing bevel gears 104 and 105. A hydraulic mechanism, which is not shown in FIG. 1, controls the pitch angle of the blades 106. Inside the hub of the propeller is a mechanical coupling 107 between the blades 106 and an inner rod or inner pipe 108, so that changing the pitch angle causes the inner pipe 108 to move back and forth in its longitudinal direction inside the propeller shaft 102, as illustrated with the arrow 109. A lever and hinge arrangement 110 converts this horizontal back and forth movement into a vertical up and down movement of a rod 111. The upper end of the rod 111, preferably equipped with a support member 112, moves an annular plate 113 up and down.
The lever and hinge arrangement 110, the rod 111, the support member 112, and the annular plate 113 are located in the azimuthally rotating part of the thruster. Attached to the fixed part, which does not move when the thruster rotates, is a vertical follower 114. The azimuthally rotating movement of the annular plate 113 does not affect the vertical follower 114, but the up and down movement of the annular plate 113—which is indicative of changes in the pitch angle—causes also the vertical follower 114 to move up and down in the direction of the arrow 115. The movements of the vertical follower 114 can be converted into an electric signal with an encoder 116. For providing a visual indication the vertical follower 114 may comprise, or it may be mechanically coupled to, a pointer 117, the position of which can be read against an adjacent scale 118.
The prior art arrangement of FIG. 1 may not give completely reliable readings, for example in cases where the annular plate 113 is not completely smooth and/or horizontal.